Calculators · Forced induction

Intercooler efficiency and supercharger pulley calculator

Two practical questions come up with every boosted engine: how well the intercooler is cooling the charge, and — on supercharged engines — how fast the blower spins with a given pulley. These calculators measure intercooler effectiveness from three temperatures, work out supercharger speed and the pulley needed for a target speed, and estimate the net power gain once charge heating and the supercharger’s own drive losses are counted.

Intercooler effectiveness

Compares how much the intercooler actually cools the air with the most it could cool it — down to ambient temperature. Good air-to-air intercoolers reach 70–85%; the density gain shows what that cooling is worth.

Results
Effectiveness—%Actual cooling divided by the maximum possible cooling.
Air density gain from cooling—%How much denser the air is after the intercooler at the same pressure.

Formula
ε = (T_in − T_out) / (T_in − T_ambient)
density gain = T_in / T_out − 1  (kelvin)

Supercharger speed from pulley sizes

A belt-driven supercharger turns faster than the engine by the ratio of the crank pulley to the blower pulley, and many centrifugal units add an internal step-up gear. This gives blower speed to check against its maximum rating.

Results
Overall drive ratio—: 1Supercharger speed per engine revolution.
Supercharger speed—rpmCompare with the manufacturer’s maximum.

Formula
ratio = (crank / blower) × step-up
blower rpm = engine rpm × ratio

Pulley size for a target supercharger speed

Works out the supercharger pulley diameter that brings the blower to a chosen speed at a given engine RPM. A smaller pulley spins the blower faster and raises boost, up to the unit’s speed limit.

Results
Supercharger pulley diameter—Pulley size that reaches the target speed.

Formula
blower pulley = crank × engine rpm × step-up / target rpm

Theoretical power gain from boost

Estimates power relative to the naturally aspirated engine from the pressure ratio and charge temperature, then subtracts the power the supercharger takes to drive. The result is an upper-end estimate that ignores knock limits and exhaust back-pressure.

Results
Gross power multiplier—× basePower from the extra air before drive losses.
Net power multiplier—× baseAfter subtracting the supercharger drive loss.

Formula
gross = PR × T_ambient / T_charge  (kelvin)
net = gross − drive loss

Worked example: a supercharged V8

  1. A 200 mm crank pulley and 80 mm blower pulley give a 2.5 : 1 drive; at 6,000 rpm the blower turns 15,000 rpm.
  2. To limit the blower to 14,000 rpm, fit an 85.7 mm pulley instead.
  3. At a pressure ratio of 1.6 with 45 °C charge air on a 25 °C day, the engine gets 1.50 times the air of the naturally aspirated version.
  4. After a 10% drive loss, the net gain is about 1.40 times base power.
  5. An intercooler cooling 115 °C air to 45 °C on a 25 °C day is 78% effective and makes the charge 22% denser.

Typical intercooler effectiveness

Intercooler typeEffectiveness
Small factory air-to-airabout 60 – 70%
Upgraded front-mount air-to-airabout 70 – 85%
Water-to-air with a good heat exchangerabout 75 – 90%
Heat-soaked air-to-air in trafficcan fall below 50%

Related

Boost pressure and pressure ratio calculatorTurbocharger airflow and power calculatorTorque, power and BMEP calculator

About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

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